1. Field of the Invention
The present invention relates to a technique for ejecting molten metal, which can be applied to a technique for ejecting solder in given amounts, for example.
2. Description of the Background Art
Techniques for ejecting solder in given amounts have conventionally been suggested. For example, Japanese Patent Application Laid-Open Nos. 62-257750 (1987) and 3-138942 (1991) disclose techniques for ejecting molten solder from a nozzle by applying pressure to the molten solder, and Japanese Patent Application Laid-Open No. 3-60036 (1991) discloses a technique for drawing out conductive paste from a nozzle with an electrostatic force.
FIG. 23 is a cross-sectional view illustrating the structure of a nozzle 200 of a device for forming solder bumps. The nozzle 200 has a molten solder chamber 1 and a straight portion 5 communicating therewith. A molten solder 20 is stored in the molten solder chamber 1. A pressure P is applied to the molten solder 20 from the side opposite to the straight portion 5 to eject a solder drop 11 from an opening 3 of the straight portion 5 at the side of the molten solder chamber 1. The nozzle 200 is made of stainless steel with poor wettability with respect to solder. This technique is disclosed in Japanese Patent Application Laid-Open No. 11-274204 (1999), for example.
FIGS. 24 to 26 are cross-sectional views showing the structure of a nozzle 201 described in Japanese Patent Application Laid-Open No. 2002-43351 by the applicant of this disclosure, where the reference characters are original to this specification. The nozzle 201 has a tapered portion 6 which spreads wider toward the molten solder chamber 1 and a straight portion 5 which communicates from the tapered portion 6 to a nozzle exit surface 4. The tapered portion 6 has edges 6a and 6b which adjoin the straight portion 5 and the molten solder chamber 1, respectively. The straight portion 5 has an opening 3 at the nozzle exit surface 4. The nozzle 201, too, is made of a solder-repellent material.
Molten solder 20 is supplied from a passage not shown and stored between the molten solder chamber 1 and a diaphragm 7 covering it. A force F is applied to the molten solder 20 from a stress source not shown, e.g. a piezoelectric device, through the diaphragm 7 that is variable in shape.
The tapered portion 6 is formed at such an angle that the molten solder 20 comes into the tapered portion 6 even when the inner surface of the nozzle 201 is solder-repellent or even when the force F is absent. For example, the tapered portion 6 is formed of the side of a frustum of a circular cone formed around an axis vertical to a bottom surface 1a of the molten solder chamber 1. The side of this frustum forms an angle α with respect to the above-mentioned axis. More specifically, when the contact angle that the molten solder 20 forms with respect to the inner surface of the nozzle 201 is taken as θs, the angle α is set to be (θs−90°) or larger. On the other hand, the inner side surface of the straight portion 5 is parallel to this axis and therefore the peripheral edge of the liquid surface of the molten solder 20 (hereinafter referred to simply as “liquid surface”) is held at the edge 6a. 
The force F pushes the molten solder 20 toward the opening 3 and then draws it back into the molten solder chamber 1. With the application of the force F in the reciprocating directions, as shown in FIG. 25, a portion of the molten solder 20 is ejected out of the nozzle 201 as a solder drop 11 through the straight portion 5 and from the opening 3.
Before being ejected, the peripheral edge of the liquid surface of the molten solder 20 is held at the edge 6a; however, after being ejected, in reaction to the ejection of the solder drop 11, it is drawn back past the edge 6a into the tapered portion 6 (FIG. 26).
Even when the nozzle is made of an easy-to-chip member, e.g., a ceramic, the nozzle 201, having the shorter straight portion 5, can be manufactured by a simpler process than the nozzle 200 having no tapered portion 5.
However, in the nozzle 201, as in the nozzle 200, the inner surface of the molten solder chamber 1 is solder-repellent and therefore the molten solder 20 exhibits poor wettability for the molten solder chamber 1. Accordingly, as shown in FIG. 27, when the molten solder chamber 1 is first charged with the molten solder 20, voids 40 may remain in part of the molten solder chamber 1. Then the voids 40 will be compressed when the diaphragm 7 presses the molten solder chamber 1, which may reduce the pressure applied to the molten solder 20 or cause a time delay, making it difficult to obtain the expected ejecting performance.